This has turned into a mess.......can't tighten dome enough

I had to replace the dome cap on the old Delta bath valve. The new one is Rp50, the replacement for my old one. Everything worked out fine, except the 'collar' behind the dome cap, the chrome piece that covers the valve body, is loose. The dome cap seems right, the lever feels right (the new cam has no adjustment ring), and it seats fully from what I can tell.
I don't get it. Is there a bigger o-ring that has to go behind the 'sleeve' or 'collar'? it's just enough of a loose fit that I know it isn't right.
Help. I am NOT a plumber....